Air pollution creates a negative externality-a cost suffered by a third party as a result of an economic transaction. A standard solution to a negative externality is a Pigouvian tax, a tax that raises the marginal private cost of pollution emissions to the level of the marginal social cost. The socially optimal quantity of pollution emissions is then determined by the intersection of the marginal private beneift, or demand, curve and the marginal social cost curve. The article notes that "putting a dollar value on the benefits of cleaner air has been difficult." Assuming this problem has been resolved, in the accompanying diagram, move the endpoints of line Smarginal social cost to show the marginal social cost curve. Then move the line labeled "Tax" to show the amount of the tax needed to limit emissions to the socially optimal level. S marginal social cost S marginal private cost Benefits and costs Tax D marginal private benefit Quantity of pollution emissions